LPP type extreme ultra violet light source apparatus and driver laser for the same

This is a divisional application of Ser. No. 11/242,016, filed Oct. 4, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an LPP (laser produced plasma) type EUV (extreme ultra violet) light source apparatus for generating extreme ultra violet light to be used for exposure of semiconductor wafers or the like. Further, the present invention relates to a driver laser for applying light to a target in the LPP type EUV light source apparatus.

2. Description of a Related Art

Recently, as semiconductor processes become finer, the photolithography has been making rapid progress to finer fabrication, and, in the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. For example, in order to fulfill the requirement for microfabrication of 50 nm or less, the development of exposure equipment with a combination of an EUV light source for generating ultra violet light of about 13 nm in wavelength and a reduced projection catoptric system is expected.

In such an EUV light source apparatus, generally, a short-pulse laser is used as a driving light source (driver), because the short-pulse laser is suitable for obtaining high CE (conversion efficiency from applied laser light to EUV light) in an LPP type EUV light source apparatus.

<figref idref="DRAWINGS">FIG. 10</figref> is a schematic diagram showing a constitution of an oscillation amplification type laser to be used as a driver.

An oscillation amplification type laser <b>10</b> shown in <figref idref="DRAWINGS">FIG. 10</figref> includes an oscillator <b>11</b> formed by a short-pulse CO₂laser and an amplifier <b>12</b> for amplifying a laser beam generated by the short-pulse CO₂laser. Here, in the case where the amplifier <b>12</b> has no light resonator, a laser system having such a constitution is referred to as “MOPA (Master Oscillator Power Amplifier) system”. The amplifier <b>12</b> has a discharging device that excites by discharge a CO₂laser gas containing carbon dioxide (CO₂), nitrogen (N₂), helium (He), and further containing hydrogen (H₂), carbon monoxide (CO), xenon (Xe), etc. according to need.

In the case where a resonator is provided in the amplification stage unlike the amplifier <b>12</b> shown in <figref idref="DRAWINGS">FIG. 10</figref>, laser oscillation can be performed by the single amplification stage. A laser system having such a constitution is referred to as “MOPO (Master Oscillator Power Oscillator) system”.

The laser beam having energy “A” output from the oscillator <b>11</b> is amplified into a laser beam having desired energy “B” in the amplifier <b>12</b>. The laser beam having energy “B” is collected through a laser beam propagation system or a lens, and applied to an EUV emission target material selected from among tin (Sn), xenon, etc.

In <figref idref="DRAWINGS">FIG. 10</figref>, only one stage of amplifier is provided for amplifying the laser energy “A” to laser energy “B”. However, when desired laser energy “B” is not obtained, plural stages of amplifiers may be used.

Next, a constitutional example of short-pulse CO₂laser as an oscillator will be described. In FIG. 5 of U.S. Pat. No. 6,697,408 discloses a constitution of a short-pulse RF (Radio Frequency) excitation CO₂laser. In this short-pulse RF-CO₂laser, highly repetitive operation of laser pulses can be performed to about 100 kHz. Practically, it is necessary to obtain EUV emission of 100 W class, however, when CE by the CO₂laser is estimated at 0.5% and propagation loss is estimated at 70%, the output required for the CO₂laser is about 60 kW. In order to achieve the output of 60 kW in the short-pulse laser, the repetition frequency of 50 kHz to 100 kHz is required when considering the durability etc. of optical elements and so on.

The reason is as below. Assuming that the output energy of the CO₂laser is E_total, the repetition frequency of pulse oscillation is f_i(i=1, 2, 3, . . . ), and the light energy of a single pulse is E_pj(j=1, 2, 3, . . . ), there is a relationship as E_total=f₁×E_p1=f₂×E_p2. Here, if E_pis larger, the damage on optical elements through which the laser beam is transmitted becomes also larger, and thereby, the optical elements deteriorate rapidly. Accordingly, E_pis desirably smaller. Therefore, to obtain the desired E_total, E_pmay be made smaller and the frequency f_imay be made larger.

In order to realize such high repetition operation, an RF (Radio Frequency) excitation CO₂laser is suitably used. The reason is that, although there is a TEA (Transverse Excitation Atmospheric) CO₂laser as another pulsed CO₂laser, the operation at about 2 kHz is a limit in the current technology.

Referring to FIG. 5 of U.S. Pat. No. 6,697,408, the laser apparatus includes a multipass waveguide laser oscillator 400 and a multipass waveguide laser amplifier 400a. A resonator of the oscillator 400 is formed by total reflection mirrors 408 and 406. A Q-switch, an RF discharge unit, and a thin film polarizer (TFP) are provided between the mirrors. When the Q-switch is OFF, a laser beam travels back and forth between the mirror 408 and the mirror 406, and light intensity increases because of simulated emission at the time. When the Q-switch is turned ON at the time when the light intensity increases sufficiently, highly peaked short pulses are reflected in the TFP 404 and directed into the multipass waveguide laser amplifier 400a shown in the lower part of FIG. 5 via a mirror 409 and a half-wave plate. Then, the introduced light is amplified in an amplifier, and a laser beam is extracted to the outside. The laser having such a constitution is referred to as “Q-switched cavity-dumped laser”.

Further, related technologies are described in the following documents (1) to (5). <ul id="ul0001" list-style="none"><li id="ul0001-0001" num="0017">(1) Fumihiko Kannari, “Numerical Simulation on the Amplification of Picosecond Pulses in Multiatmosphere CO₂Laser Media”, The Review of Laser Engineering, No. 17, No. 2, pp. 45-59, Received on Aug. 18, 1988.</li><li id="ul0001-0002" num="0018">(2) A. Endoh et al., “Temperature control of multiline oscillation of a TEA CO₂laser”, Journal of Applied Physics, 50(8), August 1979, pp. 5176-5178.</li><li id="ul0001-0003" num="0019">(3) I. V. Pogorelsky et al., “Subnanosecond Multi-Gigawatt CO₂laser”, IEEE Journal of Quantum Electronics, Vol. 31, No. 3, March 1995, pp. 556-566.</li><li id="ul0001-0004" num="0020">(4) F. Rotermund et al., “Difference-frequency generation of intense femtosecond pulses in the mid-IR (4-12 μm) using HgGa₂S₄and AgGaS₂”, Optics Communications 185, 2000, pp. 177-183.</li><li id="ul0001-0005" num="0021">(5) V. Petrovet al., “Generation of high-power femtosecond light pulses at 1 kHz in the mid-infrared spectral range between 3 and 12 μm by second-order nonlinear processes in optical crystals”, Journal of Optics A: Pure and Applied Optics 3, 2001, R1-R19.</li></ul>

As in conventional technologies, in the RF-CO₂short-pulse laser using Q-switching, since the laser gas pressure is low (generally, 40 Torr to 100 Torr), broadening of each CO₂laser transition is not sufficient and the gain spectrum is modulated in a comb-like pattern. That is, in the case where the laser gas is at low pressure, as shown in <figref idref="DRAWINGS">FIG. 11</figref>, the oscillation spectrum of CO₂laser becomes not continuous but discrete. As a result, as disclosed in the document (1), a gain in the amplification process by simulated emission, saturation thereof and a pulse shape depend on a degree of matching between a spectrum of the laser pulse and a gain spectrum.

By the way, in the case where the output light of the oscillation stage laser is directed into the amplifier in the subsequent stage and amplified as in the above-mentioned MOPA or MOPO system laser, the output spectrum of the oscillation stage laser largely affects the amplification efficiency. For example, if the oscillation stage laser has a single spectrum, that is, light energy is concentrated within a specific narrow wavelength band, the gain becomes readily saturated in the amplification stage. In the case where such a single-spectrum laser beam is input to the amplification stage of the above-mentioned MOPA or MOPO system laser and amplified, since the amplifiable spectrum is limited to one, the amplification efficiency in the multi-spectrum (multi-line) amplifiable amplification stage becomes lower. That is, since only one spectrum contributes to amplification and the rest of the spectrums cannot be laser-oscillated and contribute to amplification, and thereby, a great part of gain is wasted. This means that high output is not obtained in the amplification in the amplification stage.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems, a driver laser for extreme ultra violet light source apparatus according to one aspect of the present invention comprises: a short-pulse multi-line oscillated CO₂laser oscillator having a device that shortens width of pulses included in a laser beam to be output and a device that suppresses amplitude of an oscillation spectrum exhibiting an energy peak value; and at least one amplifier that inputs the laser beam output from the short-pulse multi-line oscillated CO₂laser oscillator and amplifies the input laser beam to output the amplified laser beam.

Further, an LPP type extreme ultra violet light source apparatus according to one aspect of the present invention comprises: a driver laser for the extreme ultra violet light source apparatus including a short-pulse multi-line oscillated CO₂laser oscillator having a device that shortens width of pulses included in a laser beam to be output and a device that suppresses amplitude of an oscillation spectrum exhibiting an energy peak value, and at least one amplifier that inputs the laser beam output from the short-pulse multi-line oscillated CO₂laser oscillator and amplifies the input laser beam to output the amplified laser beam; a target supplier that supplies a target material; and an optical system that guides the laser beam output from the driver laser such that the target material supplied from the target supplier is irradiated with the laser beam.

According to the present invention, by suppressing amplitude of the oscillation spectrum that exhibits an energy peak value of the laser beam, the oscillation of other spectrums is promoted and the laser beam is short-pulsed, and therefore, short-pulsed and multi-spectrum oscillation can be performed. As a result, the amplification efficiency in the amplifier can be increased and the short-pulsed laser beam can be amplified to a higher level.

BRIEF DESCRIPTION OF THE DRAWINGS

<figref idref="DRAWINGS">FIG. 1</figref> is a schematic diagram showing an outline of an LPP type EUV light source apparatus to which a driver laser for extreme ultra violet light source apparatus according to the present invention is applied;

<figref idref="DRAWINGS">FIG. 2</figref> is diagram for explanation of a principle of suppression of high-intensity oscillation spectrum components and smoothing of oscillation spectrum intensity by an etalon;

<figref idref="DRAWINGS">FIG. 3A</figref> is a schematic diagram showing a first constitutional example of the driver laser according to the first embodiment of the present invention, and <figref idref="DRAWINGS">FIG. 3B</figref> is a diagram for explanation of an operation of a short-pulsing device in the first constitutional example;

<figref idref="DRAWINGS">FIGS. 4A and 4B</figref> are schematic diagrams showing a second constitutional example of the driver laser according to the first embodiment of the present invention;

<figref idref="DRAWINGS">FIG. 5A</figref> is a schematic diagram showing a third constitutional example of the driver laser according to the first embodiment of the present invention, and <figref idref="DRAWINGS">FIG. 5B</figref> is a diagram for explanation of an operation of a short-pulsing device in the third constitutional example;

<figref idref="DRAWINGS">FIG. 6A</figref> is a schematic diagram showing a fourth constitutional example of the driver laser according to the first embodiment of the present invention, and <figref idref="DRAWINGS">FIG. 6B</figref> is a diagram for explanation of an operation of a short-pulsing device in the fourth constitutional example;

<figref idref="DRAWINGS">FIGS. 7A and 7B</figref> are schematic diagrams showing a fifth constitutional example of the driver laser according to the fifth embodiment of the present invention;

<figref idref="DRAWINGS">FIG. 8</figref> is a schematic diagram showing a constitution of a driver laser for extreme ultra violet light source apparatus according to the second embodiment of the present invention;

<figref idref="DRAWINGS">FIG. 9</figref> is a schematic diagram showing a constitutional example of a spectrum matching box shown in <figref idref="DRAWINGS">FIG. 8</figref>;

<figref idref="DRAWINGS">FIG. 10</figref> is a schematic diagram showing a constitution of an oscillation amplification type laser; and

<figref idref="DRAWINGS">FIG. 11</figref> shows an oscillation spectrum at the time of low gas pressure operation of CO₂laser and a pulse waveform at that time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will be described in detail by referring to the drawings. The same reference numerals are assigned to the same component elements and the description thereof will be omitted.

<figref idref="DRAWINGS">FIG. 1</figref> is a schematic diagram showing an outline of an LPP type EUV light source apparatus to which a driver laser for extreme ultra violet light apparatus (hereinafter, also simply referred to as “driver laser”) according to the present invention is applied. As shown in <figref idref="DRAWINGS">FIG. 1</figref>, the LPP type EUV light source apparatus includes a driver laser <b>1</b>, an EUV light generating chamber <b>2</b>, a target supplier <b>3</b>, and an optical system <b>4</b>.

The driver laser <b>1</b> is an oscillation amplification type laser device that generates a laser beam for driving to be used for exciting a target material. The constitution of the drive laser <b>1</b> will be described later in detail.

The EUV light generating chamber <b>2</b> is a vacuum chamber in which EUV light is generated. In the EUV light generating chamber <b>2</b>, a window <b>21</b> is provided for transmitting a laser beam <b>6</b> generated from the driver laser <b>1</b> through into the EUV light generating chamber <b>2</b>. Further, within the EUV light generating chamber <b>2</b>, a target injection nozzle <b>31</b>, a target collection tube <b>32</b> and a collection mirror <b>8</b> are provided.

The target supplier <b>3</b> supplies a target material to be used for generating EUV light through the target injection nozzle <b>31</b> as a part of the target supplier <b>3</b> into the EUV light generating chamber <b>2</b>. Within the supplied target material, a part of the material applied with no laser beam and no longer required is collected by the target collection tube <b>32</b>. As target materials, various known materials can be used. Further, status of the target material may be any of solid, liquid, or gas, and they may be supplied to space within the EUV light generating chamber <b>2</b> in any known form such as continuous flow (target jet) and droplets. For example, in the case where a liquid xenon (Xe) target is used as the target material, the target supplier <b>3</b> is formed by a compressed gas cylinder for supplying a high-purity xenon gas, a mass flow controller, a cooling device that liquefies the xenon gas, a target injection nozzle, etc. Further, when droplets are produced, a vibrating device such as a piezoelectric element is added to the constitution including them.

The optical system <b>4</b> includes a collection lens, for example, and collects the laser beam <b>6</b> output from the driver laser <b>1</b> such that a focal point may be formed in an orbit of the target material. Thereby, a target material <b>5</b> is excited to form plasma and EUV light <b>7</b> is generated.

The collection mirror <b>8</b> is a concave mirror having an Mo/Si film that reflects light of 13.5 nm with high reflectance formed on a surface thereof, and guides the generated EUV light <b>7</b> to a transmission optical system by reflecting and collecting the light. Further, the EUV light is guided to an exposure apparatus or the like via the transmission optical system. In <figref idref="DRAWINGS">FIG. 1</figref>, the collection mirror <b>8</b> collects EUV light toward the front of the paper.

Next, a driver laser according to the first embodiment of the present invention will be described. The embodiment is characterized by using a short-pulse multi-spectrum (multi-line) CO₂laser as an oscillation stage laser in the driver laser <b>1</b> (oscillation amplification type laser device). Since the use of the short-pulse multi-spectrum CO₂laser enables utilization of amplification ability in the amplification stage with high efficiency, high-power short pulses can be obtained by the amplification stage. The oscillation amplification type laser with such a short-pulse multi-spectrum CO₂laser as the oscillation amplification is used as a laser beam application source in the LPP type EUV light source apparatus for generating EUV light by applying a laser beam to a target material of xenon (Xe), tin (Sn), or the like. Thereby, a high-energy short-pulsed laser beam is applied to a target material and conversion efficiency from the applied laser beam into EUV light becomes higher, and it is useful to achieve the above-mentioned objects for realizing 100 W-class EUV light generation.

Further, in the embodiment, control of oscillation spectrum is performed in order to perform multi-line oscillation using a short-pulse CO₂laser. <figref idref="DRAWINGS">FIG. 2</figref> is a diagram for explanation of a principle of suppression of high-intensity oscillation spectrum components and smoothing of oscillation spectrum intensity performed in the short-pulse multi-spectrum CO₂laser.

Here, in a laser beam, plural oscillation spectrum components having different intensity from one another exist. As shown in <figref idref="DRAWINGS">FIG. 2(</figref><i>a</i>), in a CO₂laser beam, there is a peak in spectrum component P(<b>20</b>). If it is possible that the amplitude of this spectrum component P(<b>20</b>) is suppressed and oscillation of other spectrum components P(<b>18</b>), P(<b>16</b>), P(<b>14</b>) etc. are promoted, multi-spectrum (multi-line) oscillation can be realized. For this purpose, for example, oscillation of other spectrum components P(<b>18</b>), P(<b>16</b>), P(<b>14</b>) etc. may be promoted by inserting an optical element having loss dependent on a spectrum into the resonator of the short-pulse multi-spectrum CO₂laser. As the optical element having loss dependent on a spectrum, for example, an etalon is used. Adjustment of the etalon enables multi-line oscillation. As a material of the etalon, a material having high transmittance for a wavelength of 10.6 μm such as ZnSe and Irtran-2 is desirably used.

Accordingly, as a device that suppresses the amplitude of the oscillation spectrum exhibiting the energy peak value, an etalon having such wavelength-dependent transmittance is inserted into the resonator. Then, while the intensity of spectrum components with high peak intensity is suppressed, the etalon is adjusted such that the intensity of the spectrum components with low peak intensity may not become lower. Thereby, as shown in <figref idref="DRAWINGS">FIG. 2(</figref><i>b</i>), the intensity of oscillation spectrum can be smoothed. That is, making the intensity oscillation spectrums nearly equal enables amplification of spectrum components initially with low oscillation intensity.

In the embodiment, transmittance is controlled such that the etalon exhibits the wavelength-dependent transmission characteristic shown by the broken line in <figref idref="DRAWINGS">FIG. 2(</figref><i>a</i>) by adjusting inclination of the etalon relative to the optical axis or temperature thereof.

Next, a specific constitution for realizing a short-pulse multi-spectrum CO₂laser will be described by referring to <figref idref="DRAWINGS">FIGS. 3A to 7B</figref>.

<figref idref="DRAWINGS">FIG. 3A</figref> is a schematic diagram showing a first constitutional example of the driver laser according to the embodiment. As shown in <figref idref="DRAWINGS">FIG. 3A</figref>, the driver laser includes a short-pulse multi-spectrum CO₂laser having a laser medium <b>100</b>, a rear mirror <b>101</b> and a front mirror <b>102</b> forming a resonator, an etalon <b>103</b>, polarizing beam splitters <b>104</b> and <b>106</b>, and a Pockels cell (PC) <b>105</b>, and an amplifier <b>200</b> for inputting a short-pulsed laser beam output from the short-pulse multi-spectrum CO₂laser and amplifying and outputting it.

A seed laser passes through the laser medium <b>100</b> while traveling between the rear mirror <b>101</b> and the front mirror <b>102</b> back and forth, and CW (continuous wave)-excited or pulse-excited. At that time, as described above, suppression and amplification of oscillation intensity are performed by the etalon <b>103</b> on plural spectrum components contained in the laser beam.

The polarizing beam splitters <b>104</b> and <b>106</b> and the Pockels cell (PC) <b>105</b> form a device that shortens width of pulses included in the laser beam to be output.

The polarizing beam splitters <b>104</b> and <b>106</b> separate incident light into p-polarized light and s-polarized light by outputting p-polarized light in the same direction as the traveling direction of the incident light and output s-polarized light in a direction (upward in the drawing) nearly perpendicular to the incident light.

Further, the Pockels cell (Q-switch) is an optical element utilizing EO (electro optic) effect that the refractive index and anisotropy of a crystal change by applying an electric field to the crystal. Control of the electric field applied to the Pockels cell enables the rotation of the polarization plane of the light transmitted through the cell to a desired angle. In the constitutional example, the polarization plane of the light is rotated by π/2 (90°) due to the Pockels cell <b>105</b>. That is, at each time when the light is transmitted through the activated Pockels cell, p-polarized light becomes s-polarized light and s-polarized light becomes p-polarized light.

The laser beam output to the outside of the resonator formed by the rear mirror <b>101</b> and the front mirror <b>102</b> is cut into a desired pulse width by activating and deactivating the Pockels cell <b>105</b> with predetermined timing. Thereby, as shown in <figref idref="DRAWINGS">FIG. 3B</figref>, the multi-line-oscillated laser beam can be short pulsed.

<figref idref="DRAWINGS">FIGS. 4A and 4B</figref> show a second constitutional example of the driver laser according to the embodiment. In the constitutional example, a laser beam is short-pulsed within a resonator of a short-pulse multi-spectrum CO₂laser.

As shown in <figref idref="DRAWINGS">FIG. 4A</figref>, in the short-pulse multi-spectrum CO₂laser in the constitutional example, compared to the constitution shown in <figref idref="DRAWINGS">FIG. 3A</figref>, a resonator is formed by a rear mirror <b>101</b> and an output coupler <b>110</b>, and further, a polarizing beam splitter <b>104</b>, a Pockels cell <b>105</b>, and a quarter-wave plate <b>111</b> are provided within the resonator. These polarizing beam splitter <b>104</b>, Pockels cell <b>105</b>, and quarter-wave plate <b>111</b> form a device that shortens width of pulses included in the laser beam to be output. In the constitutional example, the polarization plane of light is rotated by π/4 (45°) due to the Pockels cell <b>105</b>. That is, at each time when the light is transmitted through the activated Pockels cell back and forth, p-polarized light becomes s-polarized light and s-polarized light becomes p-polarized light. Further, the quarter-wave plate <b>111</b> rotates the polarization plane of the light transmitted through the plate by π/4 (45°).

Further, in the short-pulse multi-spectrum CO₂laser shown in <figref idref="DRAWINGS">FIG. 4B</figref>, compared to the constitution shown in <figref idref="DRAWINGS">FIG. 4A</figref>, a resonator is formed by providing a high reflectance (HR) mirror <b>112</b> in place of the output coupler <b>110</b>. Further, in <figref idref="DRAWINGS">FIG. 4B</figref>, switching of the Pockels cell <b>105</b> is controlled to take out the laser beam upward in the drawing, and accordingly, a reflection mirror <b>113</b> for changing the direction of the taken out laser beam is provided. Such a constitution is referred to as “Q-switched cavity-dumped laser”.

As in the constitutional example, in the case where the laser beam multi-line-oscillated through the etalon <b>103</b> is short-pulsed within the resonator by the switching control of the Pockels cell <b>105</b>, there is an advantage as below. That is, since there is no longer any decrease in energy corresponding to the cut off part when the pulses are cut out outside of the resonator by oscillating the laser beam while short-pulsing it, the oscillation efficiency becomes higher.

<figref idref="DRAWINGS">FIG. 5A</figref> is a schematic diagram showing a third constitutional example of the driver laser according to the embodiment. In the constitutional example, an optical switching element is used as a device that shortens width of pulses included in a laser beam to be output from the short-pulse multi-spectrum CO₂laser.

As shown in <figref idref="DRAWINGS">FIG. 5A</figref>, in the short-pulse multi-spectrum CO₂laser in the constitutional example, a germanium (Ge) Brewster plate (germanium mirror) <b>120</b> is provided as the optical switching element in place of the polarizing beam splitters <b>104</b> and <b>106</b> and Pockels cell <b>105</b>. Further, a short-pulse laser device (e.g., Nd:YAG laser) <b>121</b> that generates a short-pulsed laser beam to be applied to the Ge Brewster plate <b>120</b> is provided.

The Ge Brewster plate <b>120</b> is normally provided so as to form a Brewster angle relative to the optical axis of the laser beam output from the resonator. When the Ge Brewster plate <b>120</b> is applied with a laser beam, the refractive index of the plate surface changes during the application of the laser beam. As a result, the Brewster angle of the Ge Brewster plate <b>120</b> also changes and the laser be an output from the resonator is reflected. Accordingly, the Ge Brewster plate <b>120</b> is applied with a short-pulsed laser beam having a desired width, and thereby, the multi-line-oscillated laser beam can be short-pulsed as shown in <figref idref="DRAWINGS">FIG. 5B</figref>.

<figref idref="DRAWINGS">FIG. 6A</figref> is a schematic diagram showing a fourth constitutional example of the driver laser according to the embodiment. In the constitutional example, a plasma shutter is used as a device that shortens width of pulses included in a laser beam to be output from the short-pulse multi-spectrum CO₂laser.

As shown in <figref idref="DRAWINGS">FIG. 6A</figref>, the short-pulse multi-spectrum CO₂laser in the constitutional example includes a rear mirror <b>101</b> and an output coupler <b>130</b> forming a resonator, a laser medium <b>100</b> and an etalon <b>103</b> provided within the resonator, and a collection optical system (e.g., collection lens) <b>131</b> for collecting a laser beam.

A laser beam oscillated within the resonator is output to the outside of the resonator, the laser beam is collected by the collection optical system <b>131</b>, and plasma <b>132</b> is generated in the gas. Since the plasma <b>132</b> absorbs the laser beam, the latter part of the laser pulse is absorbed and, as a result, the laser pulse is cut out to be short-pulsed as shown in <figref idref="DRAWINGS">FIG. 6B</figref>. The gas (plasma gas) may be air or may be helium (He), nitrogen (N₂), argon (Ar), or the like. In the latter case, plasma may be generated within a gas cell filled with the plasma gas.

<figref idref="DRAWINGS">FIGS. 7A and 7B</figref> are schematic diagrams showing a fifth constitutional example of the driver laser according to the embodiment. In the short-pulse multi-spectrum CO₂laser shown in the above-mentioned first to fourth constitutional examples, multi-line oscillation of a laser beam is performed using the etalon, however, an optical element such as a grating and prism or a combination of them may be used as wavelength splitting means (multi-line oscillation device) in place of the etalon.

For example, in <figref idref="DRAWINGS">FIG. 7A</figref>, a resonator is formed by a grating <b>140</b> and a front mirror <b>102</b>. In such a constitution, the oscillation wavelength, i.e., the intensity of oscillation spectrum can be adjusted by rotating the grating. Alternatively, a prism may be provided in place of the grating <b>140</b> shown in <figref idref="DRAWINGS">FIG. 7A</figref>.

Further, as shown in <figref idref="DRAWINGS">FIG. 7B</figref>, a resonator may be formed by a rear mirror <b>150</b> and a front mirror <b>102</b>, and a prism <b>151</b> may be provided within the resonator. In this case, the oscillation wavelength can be adjusted by rotating both the rear mirror <b>150</b> and the prism <b>151</b>.

Such a constitution using a grating and prism as wavelength splitting means can be applied to the first to fourth constitutional examples.

Next, a driver laser according to the second embodiment of the present invention will be described by referring to <figref idref="DRAWINGS">FIGS. 8 and 9</figref>.

<figref idref="DRAWINGS">FIG. 8</figref> is a schematic diagram showing a constitution of the driver laser according to the embodiment. The driver laser includes a solid-state laser <b>160</b>, a nonlinear crystal A: <b>161</b>, a nonlinear crystal B: <b>162</b>, and an amplifier <b>300</b>. The embodiment is characterized by using the solid-state laser <b>160</b> for oscillating short pulses as the oscillation stage laser in the driver laser shown in <figref idref="DRAWINGS">FIG. 1</figref>. Further, a CO₂laser is used as the amplification stage laser (amplifier <b>300</b>).

As shown in <figref idref="DRAWINGS">FIG. 8</figref>, in the embodiment, in order to realize multi-transverse mode oscillation in the wavelength region around 10 μm, two kinds of nonlinear crystals (nonlinear crystal “A”: <b>161</b> and nonlinear crystal “B”: <b>162</b>) as optical parametric oscillation devices are used with the solid-state laser <b>160</b>. Further, desirably, a spectrum matching box <b>163</b> is used in the subsequent stage of the optical parametric oscillation device.

As the solid-state laser <b>160</b>, for example, a titanium sapphire laser for oscillating in the wavelength region around 660 nm to 900 nm. Further, as a crystal of the nonlinear crystal “A”: <b>161</b>, for example, BBO (β-BaB₂O₄) is used. The nonlinear crystal “A”: <b>161</b> is phase-matched with the oscillation wavelength λ₁of the solid-state laser <b>160</b>, and thereby, according to the relationship 1/λ₁=1/λ₂+1/λ₃, a wavelength component having wavelength λ₂(wavelength component λ₂) and a wavelength component having wavelength λ₃(wavelength component λ₃) are output from the nonlinear crystal “A”: <b>161</b>. The nonlinear crystal “B”: <b>162</b> is phase-matched with those wavelength components λ₂and λ₃, and thereby, wavelength λ₄corresponding to a difference frequency between the wavelength component λ₂and the wavelength component λ₃is obtained. As the nonlinear crystal “B”: <b>162</b>, for example, AgGaS₂or HgGa₂S₂is used, and thereby, a wideband laser beam from 9 μm to 12 μm is obtained in the difference frequency. This can be realized in the case of using a laser device capable of wideband oscillation like a titanium sapphire laser. For example, non-patent document 3 and non-patent document 4 disclose, in the case where λ₁=950 nm is set, when λ₂=1400.88 nm and λ₃=1614.21 nm are set, λ₄=1060 nm is obtained.

Thus obtained light pulse having the wavelength of λ₄has a wideband, but continuous spectrum waveform. The spectrum matching box <b>163</b> converts such a light pulse into a light pulse having a comb-like spectrum waveform. This is because the gain spectrum of the CO₂laser amplifier (amplifier <b>300</b>) provided in the subsequent stage of the oscillation stage has a comb-like waveform, and, when the light pulse having a continuous spectrum waveform is input without change, it can not be amplified with efficiency.

As the spectrum matching box <b>163</b>, for example, a regenerative amplifier of CO₂laser can be used. The regenerative amplifier is formed by inserting an optical gate formed by a Pockels cell and a polarizing beam splitter into a laser resonator.

The spectrum matching box <b>163</b> shown in <figref idref="DRAWINGS">FIG. 9</figref> has a reflection mirrors <b>171</b> and <b>177</b>, Pockels cells (PCs) <b>172</b> and <b>176</b>, polarizing beam splitters <b>173</b> and <b>175</b>, and a CO₂laser medium <b>174</b>.

The reflection mirrors <b>171</b> and <b>177</b> and the CO₂laser medium <b>174</b> form a resonator.

The Pockels cells <b>172</b> and <b>176</b> are activated so as to rotate the polarization plane of light passing through them by π/4 (45°). That is, at each time when the light is transmitted through the activated Pockels cells <b>172</b> and <b>176</b> back and forth, p-polarized light becomes s-polarized light and s-polarized light becomes p-polarized light. Further, the polarizing beam splitters <b>173</b> and <b>175</b> reflect the incident s-polarized light in a direction at an angle nearly perpendicular to the incident direction and passes through the incident p-polarized light without change. These Pockels cell <b>172</b> and polarizing beam splitter <b>173</b> form an input optical gate and the Pockels cell <b>176</b> and polarizing beam splitters <b>175</b> form an output optical gate.

When a light pulse is input to the resonator, the Pockels cell <b>172</b> is activated. Then, when the light pulse is input to the polarizing beam splitter <b>173</b>, the s-polarized component of the light pulse is reflected and guided in the direction toward the Pockels cell <b>172</b>. Furthermore, the s-polarized component is transmitted through the Pockels cell <b>172</b> back and forth via the reflection mirror <b>171</b>, and its polarization plane is rotated by 90°. Afterwards, the Pockels cell <b>172</b> is deactivated, and the light pulse (p-polarized light) travels between the reflection mirror <b>171</b> and the reflection mirror <b>177</b> back and forth via the CO₂laser medium <b>174</b>.

The CO₂laser medium <b>174</b> has a comb-like gain spectrum. Accordingly, at each time when a light pulse passes through the CO₂laser medium <b>174</b>, only a wavelength component matching the comb-like gain spectrum of CO₂of continuous spectrum components that it has at the time of input to the resonator is amplified. Therefore, the light pulse is allowed to travel back and forth within the resonator to pass through the laser medium at many times, a light pulse having a comb-like spectrum (multi-spectrum, multi-line) completely matching the comb-like gain spectrum of CO₂can be generated from the continuous spectrum.

Further, when a light pulse is output from the resonator, the Pockels cell <b>176</b> is activated. Thereby, the polarization plane of the light pulse transmitted through the Pockels cell <b>176</b> back and forth via the reflection mirror <b>177</b> is rotated by 90°. As a result, the light pulse (s-polarized light) is reflected by the polarizing beam splitter <b>175</b> and output from the resonator.